Amphiphilic block copolymers are well known. The majority of amphiphilic polymers are diblock copolymers that are soluble in water. These diblock polymers are used to thicken aqueous solutions and form viscoelastic gels, such as those described in U.S. Pat. Nos. 6,506,837; 6,437,040, and US Patent application 2003/0162896.
Arkema patent application US 2006052545 describe diblock and triblock copolymer adhesives formed by a controlled radical polymerization that are capable of absorbing water and providing adhesion under humid conditions. This polymer could be water soluble.
Hydrogels are typically polymeric materials, which are capable of absorbing aqueous fluids. Hydrogels are generally hydrophilic polymeric materials that have been chemically crosslinked to remain insoluble in water (or hydrophobic chemically or physically cross-linked materials formulated with hydrocolloid materials). Hydrogels are often used in wound care, in hygiene articles, in filtration aids, as flexible cushioning materials, as control-release agents, and for the adsorption of aqueous-based fluids.
A multiblock amphiphilic block copolymer is described in US 2008/0058475, incorporated herein by reference. The non-water soluble multi-layer block has a hydrophilic middle block and hydrophobic outer blocks. It can be made by a controlled radical polymerization method. The physical properties, mechanical properties and the water absorption/transmission properties of the amphiphilic triblock copolymers described can be tailored by adjusting the levels and types of monomers in each of the block segments, as well as by adjusting the size and ratio of the blocks. Some of the properties that can be optimized include the level of water or other fluids absorbed and transmitted, gel strength, and other physical and mechanical properties. The use of ionic groups as hydrophilic monomers is disclosed and it is noted that the level of absorption in these materials is influenced by the resultant pH. In the case of acid based groups, the absorption level will increase as the relative pH increases. It is taught that this functionality can be used to control the ultimate level of absorption based on the pH of the environment. Therefore, it is the pH of the environment that controls the maximum absorption.
The amphiphilic block of the US 2008/0058475 application has excellent properties, and is useful in many applications, however, the rate of absorption for the exemplified polymers is slower than optimal in some applications. Also, due to the amphiphilic nature of the copolymers, they tend to be fairly viscous, creating issues of processibility and isolation. These processability issues become magnified when the hydrophilic segments contain ionic groups and increase with the extent of neutralization, presumably due to increased ionic interactions.
WO 2005/032610 discloses a pressure sensitive adhesive containing an amphiphillic block copolymer in a rubbery elastomeric matrix. The reference exemplifies the use of sodium acrylate in an adhesive formulation but does not describe the benefit derived from such polymer. Also, the described polymer is a diblock which is water soluble and thus falls outside the scope of this non-water soluble hydrogel disclosure and furthermore the amount of neutralized polymer is low in the formulation and thus the effect on absorption rate is minimal, presumably because the neutralized portion is not the continuous phase, i.e., matrix.
It has now been found that the absorption rate of amphiphilic multiblock polymers can be dramatically increased by neutralizing (partially or fully) the hydrophilic monomer. The neutralization extent can in part be used to control the desired absorption rate. The neutralization tends to increase the viscosity leading to difficulties in polymer processing and end-use article fabrication. Processability of the multi-layer amphiphilic block copolymers can be improved by the use of a diluent, preferably a plasticizer for the neutralized hydrophilic polymer segment. In a preferred embodiment, the diluent used is a water absorbing diluent.